1. Technical Field
The present invention relates to charging systems in general, and in particular to a method for ensuring safe use of a battery pack after an impact had occurred on the battery pack.
2. Description of Related Art
Most rechargeable batteries mounted on a notebook personal computer (PC) take a form of a battery pack that includes multiple battery cells composed of a lithium ion rechargeable battery having a high-energy density and in which the battery cells are combined by serial or parallel connection and are contained in a housing.
With reference now to the drawings, and in particular to FIGS. 10A and 10B, there are depicted are partial cross sectional views of a cylindrical cell 1000 of a lithium ion rechargeable battery. Specifically, FIG. 10A shows a normal state of the cell. A positive electrode 1003 is formed by applying lithium cobalt oxide on both surfaces of a metal foil. A negative electrode 1001 is formed by applying a carbon material on both surfaces of a metal foil. Connection terminals (not shown) are welded to the positive electrode 1003 and the negative electrode 1001. A separator 1005 is inserted between the positive electrode 1003 and the negative electrode 1001 and they are wound in a cylindrical shape and inserted into an outer container 1007. The positive electrode 1003 and the negative electrode 1001 are also connected to external terminals (not shown).
These elements are inserted into the outer container 1007 and an electrolytic solution 1009 is injected therein and hermetically sealed. The separator 1005 is a porous insulating film that allows movement of ions therethrough, and the electrolytic solution 1009 is an organic electrolytic solution in which a lithium salt is dissolved in an inflammable organic solvent. The outer container 1007 has installed therein a gas discharge value (not shown) that discharges gas generated from the decomposed electrolytic solution to prevent explosion.
Upon charging/discharging the lithium ion rechargeable battery, it is necessary to precisely control charging/discharging current and voltage. Therefore, in a battery pack using the lithium ion rechargeable battery, a microprocessor unit (MPU) is provided within the battery pack. In recent years, a battery pack generally employs a scheme called a smart battery in which an MPU monitors an internal state of the battery pack during charging and discharging to send information to a notebook PC or to activate a protection circuit. The smart battery is a battery device that is compliant with the standards called smart battery system (SBS) initiated by Duracell Inc. and Intel Inc. A battery pack compliant with the above standards is also called an intelligent battery.
In an intelligent battery, an electric circuit portion includes an MPU, a current measurement circuit, a voltage measurement circuit, a temperature sensor, and the like, and a rechargeable battery, all mounted on a substrate contained within a housing. The MPU is operable to communicate with an embedded controller of the notebook PC body via data lines. A protection circuit is also installed in the intelligent battery. When an abnormality occurs in the cell during operation, the protection circuit can be activated to stop any charging/discharging operations. In addition, the remaining capacity of the intelligent battery is monitored by the MPU; therefore, it is possible to change a power consumption mode of the notebook PC in accordance with the remaining capacity in cooperation with the notebook PC body. Moreover, when the remaining capacity becomes low or when an abnormality has occurred in the battery, an alarming message may be displayed on a display so that the operation of the notebook PC can be stopped.
The lithium ion rechargeable battery can be charged up to a rated capacity at the start of use; however, a chargeable capacity decreases with repeated charging and discharging operations. The chargeable capacity decreasing with the repeated charging and discharging operations is referred to as the full charge capacity at respective time instants. For example, the full charge capacity of the lithium ion rechargeable battery after 300 times of repeated charging and discharging becomes about 80 percents of the rated capacity.
The battery pack is usually carried while being mounted on the notebook PC rather than carried alone. The housing of the battery pack mounted on the notebook PC often constitutes a portion of a housing of the notebook PC body. In addition, a so-called extension battery pack is also employed in which a portion of the housing of a battery pack protrudes out from the housing of the notebook PC. When the notebook PC is dropped and a strong impact having the weight of the notebook PC added thereto is applied to the battery pack, there is a possibility of a short-circuiting occurring in the cell, causing an explosion or fire, which may spread into the entire cell.
As described above, an intelligent battery includes a current measurement circuit, a voltage measurement circuit, and a temperature sensor. When the cell is subjected to an impact whereby the positive electrode 1003 and the negative electrode 1001 are short-circuited to cause an electrical abnormality such as abnormally large current flow or a physical abnormality such as temperature rise in a specific cell, the intelligent battery can detect such abnormalities and activates a protection circuit. Such an internal protection circuit of the battery pack is only able to cope with a current increase or temperature rise that progresses relatively slowly with the abnormalities of the cell generated by the impact; however, it cannot perfectly protects the battery pack from an abrupt phenomenon such as an explosion or fire.
FIG. 10B shows a state where the cell 1000 is subjected to an impact from the direction of an arrow X and a depression (dent) 1011 is formed in a portion of the outer container 1007. Depending on the level or type of the impact applied to the outer container 1007, the depression 1011 formed in the outer container 1007 may be subtle. In such a case, the electrolytic solution 1009 or the inflammable gas may not be discharged outside, and moreover, no prominent change in the temperature or current may be detected by a sensor. However, when the cell 1000 is continuously charged and discharged in such a state, a damage 1013 as illustrated in FIG. 10B may occur inside the cell 1000, such as breakage of the positive electrode 1003, the negative electrode 1001, and the separator 1005; abnormally narrow gap or subtle short-circuiting between the positive electrode 1003 and the negative electrode 1001; or peeling of an active material (lithium cobalt acid of the positive electrode 1003 and carbon of the negative electrode 1001) on the electrode surface.
Moreover, whenever charging/discharging operations are repeated, a physical change may occur inside the cell 1000, such as repeated expansion/contraction of the positive electrode and the negative electrode or adhering of lithium metal on the negative electrode 1001. As a result, when the charging/discharging operations are repeatedly performed on the cell 1000 having been subjected to the impact, the damage 1013 may develop, leading into a completely short-circuited state and immediately generating heat, whereby the electrolytic solution 1009 or the inflammable gas may be discharged from the outer container 1007. Therefore, when the battery pack is subjected to an impact which is expected to cause the battery pack to fall into a dangerous state, it is desirable to stop the use and to replace with a new product even if no abnormal current increase or temperature rise is detected.
According to the prior art, an impact applied to the battery pack is detected using an impact sensor or the like, and the battery pack having been subjected to the impact is immediately prohibited from use. In conjunction, an alarm message is sent to a user to prompt replacement of the battery pack.
However, when a user having received such an alarm message is unable to immediately replace the battery pack, the portability of the notebook PC and the user's convenience may be impaired. Moreover, it is difficult to precisely set a use condition or period that guarantees a safe use after occurrence of the impact. A battery pack having been subjected to an impact greater than a predetermined magnitude is more likely to catch fire as the period of continued use increases; however, the danger level also depends on the magnitude or frequency of the impact. Although it is not necessary to immediately prohibit the use of a notebook PC, in view of the user's convenience or safety, it is appropriate to immediately prohibit the use of the battery pack in a state where it is undesirable to continue use for a long time, or to leave the battery pack without applying any restriction.
Consequently, it would be desirable to provide a method for ensuring safe use of a battery pack after an impact.